Index contains unexpected zero page at block

From time to time I get this and similar errors in my Postgres log file:

< 2015-12-17 07:45:05.976 CST >ERROR: index
"user_pictures_picture_dhash_idx" contains unexpected zero page at block
123780
< 2015-12-17 07:45:05.976 CST >HINT: Please REINDEX it.
< 2015-12-17 07:45:05.976 CST >CONTEXT: PL/pgSQL function
select_pictures_by_dhash(bigint,integer,integer) line 3 at RETURN QUERY
< 2015-12-17 07:45:05.976 CST >STATEMENT: SELECT * FROM
select_pictures_by_dhash(8559245352688506666,100,0)

I have tried to recreate the index (create new index, drop the old one),
but it doesnt help, the error comes back anyway. Also, its not there all
the time, when I tried to rerun the statement from the logfile above a
couple of hours later it worked fine without any error.

The server is a read slave, set up with streaming replication. We run
PostgreSQL 9.3.5.

Is this anything I should be worried about, and if so, what can I do to fix
it? Will it be fixed with a newer version of Postgres?

Thanks!
Victor